Figure 5

Molecular model of the human NaCT protein and localization of amino acids investigated in this study. (A) Molecular model of human NaCT indicating sites of investigated mutations. One of the subunits is shown as grey ribbon and the other subunit is shown in color gradient ranging from blue (N-terminus) to red (C-terminus). In both subunits, a citrate molecule is shown in stick presentation (atom-type coloring). The grey subunit additionally shows the position of the residues investigated by side-directed mutagenesis. These residues are shown in space-filled presentation using the following color code: G219 (yellow), T227 (orange), D243 (cyan), L420 (dark blue), L485 (red), L488 (green) and F500 (light blue). (B) Same presentation as in (A) but rotated by 90° around the horizontal axis. This corresponds to a view on the NaCT protein from the extracellular site. (C,D) Effect of the T227M mutation on the NaCT structure. In the wild-type protein, T227 (orange) is close to the substrate citrate (atom-type coloring). (D) Due to its longer sidechain M227 (orange) would form clashes with a citrate, which will no longer allow the transport of this substrate. (E,F) Effect of the G219R mutation on the NaCT structure. In the wild-type protein G219 (yellow) interacts with A133 (atom-type coloring) of an adjacent helix. (F) Due to its longer sidechain R219 (yellow) would form clashes with A133 thereby disrupting the NaCT structure.